On Wed, Apr 12, 2023, Xin3 Li wrote: > > > And then this is equally gross. Rather than funnel FRED+legacy into a single > > function only to split them back out, just route FRED into its own asm subroutine. > > The common bits are basically the creation/destruction of the stack frame and > > the CALL itself, i.e. the truly interesting bits are what's different. > > I try to catch up with you but am still confused. > > Because a FRED stack frame always contains an error code pushed after RIP, > the FRED entry code doesn't push any error code. > > Thus I introduced a trampoline code, which is called to have the return > instruction address pushed first. Then the trampoline code pushes an error > code (0 for both IRQ and NMI) and jumps to fred_entrypoint_kernel() for NMI > handling or calls external_interrupt() for IRQ handling. > > The return RIP is used to return from fred_entrypoint_kernel(), but not > external_interrupt(). ... > > + /* > > + * A FRED stack frame has extra 16 bytes of information pushed at the > > + * regular stack top compared to an IDT stack frame. > > + */ > > + push $0 /* Reserved by FRED, must be 0 */ > > + push $0 /* FRED event data, 0 for NMI and external interrupts */ > > + shl $32, %rax > > + orq $__KERNEL_DS | $FRED_64_BIT_MODE, %ax > > + push %rax /* Vector (from the "caller") and DS */ > > + > > + push %rbp > > + pushf > > + push \cs_val > > We need to push the RIP of the next instruction here. Or are you suggesting > we don't need to care about it because it may not be used to return from the > callee? ... > > + push $0 /* FRED error code, 0 for NMI and external interrupts */ > > + PUSH_REGS > > + > > + /* Load @pt_regs */ > > + movq %rsp, %_ASM_ARG1 > > + > > + call \call_target The CALL here would push RIP, I missed/forgot the detail that the error code needs to be pushed _after_ RIP, not before. Unless CET complains, there's no need for a trampoline, just LEA+PUSH the return RIP, PUSH the error code, and JMP to the handler. IMO, that isn't any weirder than a trampoline, and it's a bit more obviously weird, e.g. the LEA+PUSH can have a nice big comment.
